With an exponential increase in communication demands such as mobile communication, television (TV) broadcasting, a local area network, or a metropolitan area network, limited frequency resources have almost reached a saturation state. Therefore, technologies for efficiently distributing and utilizing limited frequency resources have recently attracted attention.
Among these technologies, research into Cognitive Radio (CR) has been actively conducted, which can recognize the current condition of use of frequency bands that are continuously changed, select an available frequency band, and utilize the available frequency band without interfering with a conventional radio environment, thus maximizing the frequency usage rate.
CR is a scheme for searching a spectrum of a very wide frequency range, utilizing a currently usable frequency band, controlling a carrier frequency and bandwidth, and transmitting signals, rather than a scheme for transmitting signals using a fixed carrier frequency as in the case of a conventional communication scheme. That is, a CR system is characterized in that, after a very wide band has been searched, transmission can be performed using any frequency band.
In the conventional communication system, the transmission bandwidth (BW) of signals is generally much smaller than a carrier frequency fc (fc>>BW). Therefore, harmonic signals occurring at integer multiples of fc (2fc, 3fc, 4fc, . . . ) due to the nonlinear characteristics of a power amplifier are undesirable signals, and can be easily eliminated by a filter at the output terminal of the amplifier.
However, as described above, the CR system must take into consideration a wide band compared to the conventional communication system, and a carrier frequency and bandwidth must be freely changed in the corresponding band, so that the transmission bandwidth to be considered is much greater than the carrier frequency (fc<<BW). Therefore, the output filter of the amplifier has a wideband pass function without causing only a specific frequency to pass therethrough. Consequently, a transmittable frequency band ranges over a wide band, so that harmonic signals generated by the amplifier may fall within the corresponding communication band, thus making it difficult to eliminate the harmonic signals at the output terminal of the amplifier. That is, since the frequency of harmonic signals may be used to transfer transmission signals as occasion demands, a specific frequency filter cannot be installed. Further, in the CR system, the carrier frequency of signals desired to be transmitted is dynamically changed, so that harmonic signals are also dynamically changed, thus making it difficult to eliminate harmonic signals caused by nonlinear output signals by using a filter or the like.
When these harmonic signals are not eliminated, they influence the conventional communication system, such as by acting as interference to the conventional communication system, in a system borrowing any available spectrum, as in the case of the CR system, and thus the harmonic signals must be eliminated.